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Addition polymer structures and uses

Key idea: In addition polymerisation, alkene double bonds open so monomers join into long chains. Every monomer atom stays in the polymer; no small molecule is released.

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Plastic bags and clingfilm are made of poly(ethene), a molecule thousands of carbon atoms long built from ethene. This lesson shows how alkenes join into addition polymers, and how to move between a monomer and the repeat unit of its polymer. It relies on one idea from alkene reactions: the C = C bond can open so that new atoms join each carbon.

Monomers and polymers

A polymer is a very large molecule built up from many small units called monomers. The part of the polymer chain that repeats is its repeat unit. Different polymers are made from different monomers, so they have different repeat units and different properties.

Poly(ethene): addition polymerisation

In addition polymerisation, many alkene molecules join together. One bond of each C = C opens and the monomers bond to one another in a long chain. The polymer is the only product.

nCH₂ = CH₂ → [-CH₂-CH₂-]ₙ

Here n is a large number, often thousands. Ethene is the monomer and poly(ethene) is the polymer.

ethene forming poly(ethene)The monomer ethene: two carbons joined by a double bond. The first carbon carries two H; the second carries two H. An arrow leads to the repeat unit of poly(ethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHHCCHHHHnethenepoly(ethene)
ethene forming poly(ethene)The monomer ethene: two carbons joined by a double bond. The first carbon carries two H; the second carries two H. An arrow leads to the repeat unit of poly(ethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHHetheneCCHHHHnpoly(ethene)
The displayed repeat unit keeps both carbons and all four hydrogens. The bond through each bracket shows that the chain continues.

Poly(ethene) is a thermoplastic: suitable grades can be melted and reshaped. Grades used for plastic bags and clingfilm are flexible, waterproof, light and inexpensive, and do not readily react with food or water. Other grades can be more rigid, so choose properties appropriate to the product.

Check your understanding 1 Choose using both job requirements

A food bag must fold without breaking and keep liquid inside. Grade A is flexible and waterproof; grade B is rigid and waterproof. Both resist reaction with food. Choose the better grade and the property that distinguishes it.

Better grade for this bag
Property distinguishing the grades for this job
Show answer

Choose A for flexibility as well as water resistance. Explain why the shared properties alone cannot decide the choice in the next activity.

Check your understanding 2 Uses of poly(ethene)

A food bag needs to fold without breaking and keep liquid inside. Grade A is flexible and waterproof; grade B is rigid and waterproof. Both resist reaction with food. Choose the better grade and explain why water resistance alone is insufficient.

Work on paper or in the optional notes. Compare folding, water retention and the properties both grades share.

Show answer

Choose A: its flexibility allows folding and its water resistance keeps liquid inside. Both grades are waterproof, so that property alone cannot decide the choice. B’s rigidity does not meet the folding requirement. Resistance to reaction with food is useful for both, but also does not distinguish them.

From a monomer to a repeat unit

  1. Draw the two carbons of the C = C bond with a single bond between them.
  2. Keep every other atom or group on the same carbon it was on in the monomer.
  3. Put brackets around the two carbons, draw a bond through each bracket, and write n after the closing bracket.
propene forming poly(propene)The monomer propene: two carbons joined by a double bond. The first carbon carries two H; the second carries H and CH3. An arrow leads to the repeat unit of poly(propene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHCH₃CCHHHCH₃npropenepoly(propene)
propene forming poly(propene)The monomer propene: two carbons joined by a double bond. The first carbon carries two H; the second carries H and CH3. An arrow leads to the repeat unit of poly(propene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHCH₃propeneCCHHHCH₃npoly(propene)
The CH₃ group stays on the same carbon. Only the two carbons of the double bond are in the main chain.

Guided practice 1

Deduce a new repeat unit

About 6 min

Problem

An alkene has the structure CH₂ = CHCH₂CH₃. First identify the bond, attached group and number of backbone carbons in its repeat unit. Construct and compare the complete repeat unit in the next activity.

Track the two backbone carbons

Bond between the two carbons in the repeat unit
Group kept on the second carbon

Hints

Hint 1

find the joining carbons

Which two carbons can join neighbouring monomers?
Hint 2

show the repeat

Put the two-carbon backbone unit in brackets with a bond through each bracket and n.
Show solution step by step

  1. Open the double bond

    Method

    Join the two double-bond carbons with a single bond.

    Reason

    Addition polymerisation uses one bond of C = C to join the monomers. The other two carbons stay in the side group.

    Working

    Answer

    CH₂-CH(CH₂CH₃), with one bond free on each backbone carbon to join the next unit.

  2. Show the repeat

    Method

    Add brackets, continuation bonds and n.

    Reason

    The unit repeats along the whole chain.

    Working

    Answer

    [-CH₂-CH(CH₂CH₃)-]ₙ

Check your understanding 3 Construct the full repeat unit

For CH₂ = CHCH₂CH₃, draw the addition-polymer repeat unit on paper. Show the two backbone carbons, attached atoms/groups, brackets, continuation bonds and n. Count all carbons and hydrogens and explain which carbons are in the side group. Optional notes can record your comparison; you need not transcribe the drawing.

Show answer

The repeat unit is [-CH₂-CH(CH₂CH₃)-]ₙ. The two former double-bond carbons form the single-bond backbone. The CH₂CH₃ group remains attached to the second backbone carbon. Draw a bond through each bracket and n outside. Each repeat unit retains four C and eight H: two C are in the backbone and two in the side group. Compare the connections and continuation bonds as well as the atom counts with the model below.

given alkene forming addition polymerThe monomer given alkene: two carbons joined by a double bond. The first carbon carries two H; the second carries H and CH2CH3. An arrow leads to the repeat unit of addition polymer: the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHCH₂CH₃CCHHHCH₂CH₃ngiven alkeneaddition polymer
given alkene forming addition polymerThe monomer given alkene: two carbons joined by a double bond. The first carbon carries two H; the second carries H and CH2CH3. An arrow leads to the repeat unit of addition polymer: the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHCH₂CH₃given alkeneCCHHHCH₂CH₃naddition polymer
The two former double-bond carbons form the backbone. The two-carbon side group remains attached to its original carbon.

Spot the mistake 2

A repeat unit that keeps its double bond

About 4 min

Learner working

A learner writes the repeat unit of poly(ethene) as [-CH₂ = CH₂-]ₙ and says water is given off as the monomers join. First choose the correct backbone bond and whether another product forms. Correct the complete repeat unit and explain the atom balance in the next activity.

Track the bond and the products

Bond between the carbons in the repeat unit
Other product
Show solution step by step

  1. Count bonds on each carbon

    Method

    Count the bonds on a carbon in the learner’s repeat unit.

    Reason

    Each carbon can form only four bonds.

    Working

    Answer

    2 (to H) + 2 (double bond) + 1 (continuation bond) = 5: impossible.

  2. Correct the unit and the products

    Working

    Answer

    The repeat unit is [-CH₂-CH₂-]ₙ with a single bond. No water forms: every atom of every ethene molecule ends up in the polymer.

Check your understanding 4 Correct the complete polymer representation

A learner used [-CH₂ = CH₂-]ₙ for poly(ethene) and claimed water is released. Draw the corrected repeat unit on paper and explain why the original carbon bonds and water claim fail. Include brackets, continuation bonds and n. Optional notes can record your comparison.

Show answer

Use [-CH₂-CH₂-]ₙ with a single backbone bond and a continuation bond through each bracket. Each carbon has two H bonds, one bond to the other backbone carbon and one continuation bond: four. Keeping the double bond would make five. Every monomer atom remains in the polymer, so no water or other small molecule is released.

ethene forming poly(ethene)The monomer ethene: two carbons joined by a double bond. The first carbon carries two H; the second carries two H. An arrow leads to the repeat unit of poly(ethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHHCCHHHHnethenepoly(ethene)
ethene forming poly(ethene)The monomer ethene: two carbons joined by a double bond. The first carbon carries two H; the second carries two H. An arrow leads to the repeat unit of poly(ethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHHetheneCCHHHHnpoly(ethene)
Check all four bonds on each repeat-unit carbon, including the bond continuing through the bracket.

Check your understanding 5 Track the unfamiliar monomer’s atoms

Tetrafluoroethene is CF₂ = CF₂. In each addition-polymer repeat unit, choose the bond between these carbons and count the total carbon and fluorine atoms retained.

Backbone carbon–carbon bond
Show answer

Each unit retains two carbon and four fluorine atoms, with a single bond between its backbone carbons. Now construct the full bracketed unit and explain the bond change.

Check your understanding 6 The polymer of tetrafluoroethene

Tetrafluoroethene is CF₂ = CF₂. Write the repeat unit of its addition polymer and describe the bond change.

Work on paper or in the optional notes. Compare all atoms and attached groups, the single backbone bond, continuation bonds, brackets and n.

Show answer

[-CF₂-CF₂-]ₙ: the carbon–carbon double bond becomes single, with a continuation bond through each bracket. Each carbon keeps its two fluorine atoms. All monomer atoms remain in the polymer.

tetrafluoroethene forming poly(tetrafluoroethene)The monomer tetrafluoroethene: two carbons joined by a double bond. The first carbon carries two F; the second carries two F. An arrow leads to the repeat unit of poly(tetrafluoroethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCFFFFCCFFFFntetrafluoroethenepoly(tetrafluoroethene)
tetrafluoroethene forming poly(tetrafluoroethene)The monomer tetrafluoroethene: two carbons joined by a double bond. The first carbon carries two F; the second carries two F. An arrow leads to the repeat unit of poly(tetrafluoroethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCFFFFtetrafluoroetheneCCFFFFnpoly(tetrafluoroethene)
Every fluorine stays on its carbon; the double bond becomes single and new bonds join neighbouring units.

From a repeat unit back to the monomer

  1. Find the two carbons inside the brackets.
  2. Remove the brackets, the bonds through them and the n.
  3. Replace the single bond between the two carbons with a double bond. Every other group stays where it is.
chloroethene forming poly(chloroethene)The monomer chloroethene: two carbons joined by a double bond. The first carbon carries two H; the second carries H and Cl. An arrow leads to the repeat unit of poly(chloroethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHClCCHHHClnchloroethenepoly(chloroethene)
chloroethene forming poly(chloroethene)The monomer chloroethene: two carbons joined by a double bond. The first carbon carries two H; the second carries H and Cl. An arrow leads to the repeat unit of poly(chloroethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHClchloroetheneCCHHHClnpoly(chloroethene)
Reading right to left recovers the monomer: remove the brackets and bonds through them, then restore the double bond.

Worked example 3

Find the monomer of a polymer

Problem

A polymer has the repeat unit [-CH₂-CHCl-]ₙ. Deduce its monomer.
Show solution step by step

  1. Isolate the two carbons

    Method

    Remove the brackets, continuation bonds and n.

    Reason

    These show only that the unit repeats; they are not part of the monomer.

    Working

    Answer

    CH₂-CHCl

  2. Restore the double bond

    Method

    Make the carbon–carbon bond double.

    Reason

    Each carbon used one bond to join its neighbours in the chain; in the monomer those bonds are the second bond of C = C.

    Working

    Answer

    Monomer: CH₂ = CHCl, chloroethene.

Check your understanding 7 Recover the bond without moving groups

A polymer repeat unit is [-CHCl-CHCl-]ₙ. For its monomer, choose the bond between the two carbons and the number of chlorine atoms that stay on each carbon.

Carbon–carbon bond in the monomer
Show answer

Restore the double bond and keep one H and one Cl on each carbon. Now construct the complete monomer and explain the reverse procedure.

Check your understanding 8 Recover a monomer

A polymer has the repeat unit [-CHCl-CHCl-]ₙ. Write its monomer and explain how you recover it.

Work on paper or in the optional notes. Compare the double bond, unchanged H/Cl positions and removal of the polymer-only brackets, continuation bonds and n.

Show answer

The monomer is CHCl = CHCl. Remove the brackets, continuation bonds and n, then restore the double bond between the two backbone carbons. Each H and Cl stays on its original carbon.

Common mistakes

  • Leaving a C = C double bond in the repeat unit.
  • Putting a side group such as CH₃ or Cl into the main chain.
  • Forgetting the bonds through the brackets, or the n.
  • Restoring the monomer without its double bond.
  • Saying a small molecule such as water is given off. In addition polymerisation the monomers simply join.

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